A method and apparatus for authentication in a passive optical network are disclosed. In the disclosure, a first terminal serial number of an ONU and a first logic registration code are transmitted from the ONU to an OLT; if the OLT determines that the first terminal serial number does not match a second terminal serial number stored on the OLT, the OLT judges whether the first logic registration code received from the ONU matches a second logic registration code stored on the OLT; the OLT stores the first terminal serial number received from the ONU on the OLT if the first logic registration code matches the second logic registration code.